Shadow mask
A shadow mask (cf. stripe mask, slit mask ) is a thin metal screen, which is mounted in the color picture tube is about 20 mm behind the screen. The shadow mask has as many holes as color triplets are present on the screen to each hole includes three colored fluorescent phosphor dots. Meet The three electron beams together at the same place the mask, however, pass through the mask holes through only those electrons on the corresponding luminous point of Leuchttripels (blue, green, red), which originate from the associated electron: The three electron beams are for all three colors equally but coming from a slightly different direction, since the electron guns in the tube neck in a triangle ( delta tube or " Chromatron " ) or slit masks ( in-line or " Trinitron ") are arranged next to each other. The shadow mask shades off from the respective direction, the phosphors with the wrong colors, that is, they prevent, for example, the electron beam for red the bodies strikes the phosphor layer, the light green. The light is generated by fluorescence by the impingement of the electrons on the corresponding phosphor layers on the rear side of the screen. All three electron must take on the mask and the screen always together in the same place. The effort to achieve this convergence even when the beam deflection over the entire surface of the screen is very high in delta or delta arrangement of the electron beams.
To reduce the correction effort, the mask is, in particular in television sets of round holes instead of narrow vertical slits. One then speaks of a slit mask; the electron guns are in this case arranged side by side ( Trinitron ), the convergence requirement is reduced, it enters a higher proportion of the electrons on the phosphor screen.
Computer monitors use but still shadow masks to (eg CAD applications ) to avoid aliasing effects on straight edges.
The resolution of a picture tube is essential from the hole spacing (dot pitch) dependent. This is for monitors from 0.21 to 0.28 mm, and usually the distance between two holes ( hole center to hole center ) is specified. The smaller the hole spacing, the higher the possible resolution of the tube and all the more detailed the image display is - provided that the focusing and convergence are exact.
In the left figure the macro shot of the word is to see shadow mask from this article. The individual color components red, green and blue can also be seen very clearly. Characterized in that the human eye is unable to resolve the individual colored dots produced by additive color mixing of the impression that it is, for example, is white.
Especially on the screen of a monitor or a television picture tube, the convergence error are (the three electron beams do not hit the same place on ) visible. They lead to color edges of contours. The convergence is with shadow mask ( delta arrangement of the electron ) in contrast to the Trinitron ( beam next to each other ) difficult to achieve in monitors.
A problem of shadow masks is the so-called doming ( bulge ), which is produced by the opposite slit masks comparatively high heat input into the shadow mask and the thermal deformation.
Production
Hole and slot masks for televisions and computer monitors are made from 140 microns to about 250 microns thick invar steel or other low thermal expansion. The shadow mask sheet is coated with photoresist (eg casein). After the exposure, in a so-called " printer " is etched holes at about 70 ° C from under the action of chlorine gas. After cleaning and drying the masks extensive technical checks are carried out. Meanwhile, the production of hole and slot masks focuses largely in East Asia. The last Western European manufacturer was located in the southern Baden garbage home.
Coating the tube with the phosphors is effected by photo-lithographic process in several steps. The exposure is carried out here with exactly the steel mask, which is later incorporated exactly into this tube. Thus, a good congruence between the point grid on the glass bulb and the steel mask is achieved.
Extract raster mask